1. Field of the Invention
The present invention relates to an acoustic wave filter device for use in, for example, a duplexer in a cellular phone and that includes a plurality of band-pass filters. More specifically, the present invention relates to an acoustic wave filter device in which a plurality of acoustic wave filters having an balanced-to-unbalanced transforming function are disposed on a single piezoelectric substrate.
2. Description of the Related Art
Traditionally, a surface acoustic wave filter device is widely used as a band-pass filter at an RF stage of a cellular phone or the like. To achieve miniaturization, it is preferable that a plurality of surface acoustic wave filters be formed on a single piezoelectric substrate. With this structure, the number of parts can be reduced, and the cellular phone can be further miniaturized.
At an RF stage of a cellular phone, it is preferable that a surface acoustic wave filter device also have the balanced-to-unbalanced transforming function. With this structure, a component for performing the balanced-to-unbalanced transforming function, that is, a balun, can be omitted. Accordingly, a surface acoustic wave filter device of the longitudinally-coupled resonator type having the balanced-to-unbalanced transforming function has begun to be used as a band-pass filter at the RF stage of the cellular phone.
One example of a surface acoustic wave filter device in such a use is disclosed in WO 2006/003787A1 described below.
FIG. 19 is a schematic plan view that illustrates the surface acoustic wave filter device described in WO 2006/003787A1.
A surface acoustic wave filter device 1001 includes a piezoelectric substrate 1002. A schematically illustrated electrode structure is formed on the piezoelectric substrate 1002, thus providing a first surface acoustic wave filter 1003 having the balanced-to-unbalanced transforming function and a second surface acoustic wave filter 1004 having the balanced-to-unbalanced transforming function. That is, the plurality of surface acoustic wave filters are provided on the same piezoelectric substrate 1002. For the first surface acoustic wave filter 1003, a first end of a central first IDT 1011a of a 3-IDT first-stage longitudinally-coupled resonator type surface acoustic wave filter portion 1011 is connected to an unbalanced terminal 1005, and a second end thereof is connected to a ground potential. First ends of second IDTs 1011b and 1011c arranged at both sides of the IDT 1011a are commonly connected by a line 1013 to an external ground potential.
Second ends of the second IDTs 1011b and 1011c are connected to first ends of a pair of second IDTs 1012b and 1012c, respectively, of a 3-IDT second-stage longitudinally-coupled resonator type surface acoustic wave filter portion 1012, which is connected downstream thereof. Second ends of the IDTs 1012b and 1012c are commonly connected by the line 1013 to the ground potential. A first IDT 1012a arranged in a central part of the second-stage longitudinally-coupled resonator type surface acoustic wave filter portion 1012 is partitioned into two parts in an acoustic wave propagation direction and thus includes first and second partitioned IDT portions 1012a1 and 1012a2. First ends of the partitioned IDT portions 1012a1 and 1012a2 are connected to first and second balanced-to-unbalanced transforming functions 1006 and 1007, respectively.
Also, for the second surface acoustic wave filter 1004 formed on the piezoelectric substrate 1002, 3-IDT longitudinally-coupled resonator type surface acoustic wave filter portions 1021 and 1022 are connected between an unbalanced terminal 1008 and first and second balanced terminals 1009 and 1010.
In this case, first ends of first IDTs 1021a and 1022a arranged in central parts of the longitudinally-coupled resonator type surface acoustic wave filter portions 1021 and 1022, respectively, are commonly connected by a line 1023 to an external ground potential. The IDT 1021a is connected to the first balanced terminal 1009 through a surface acoustic wave resonator 1026. The IDT 1022a is connected to the second balanced terminal 1010 through a surface acoustic wave resonator 1027.
First ends of second IDTs 1021b and 1021c arranged at both sides of the first IDT 1021a and first ends of second IDTs 1022b and 1022c arranged at both sides of the first IDT 1022a are commonly connected to the unbalanced terminal 1008 through a surface acoustic wave resonator 1025.
For the surface acoustic wave filter device 1001 described in WO 2006/003787A1, the plurality of surface acoustic wave filters 1003 and 1004 are provided on the same piezoelectric substrate 1002. Because the plurality of filters are formed on the same piezoelectric substrate, the number of parts can be reduced, and a cellular phone or other devices in which the surface acoustic wave filter device 1001 is accommodated can be miniaturized.
Meanwhile, for the surface acoustic wave filter device 1001, because the plurality of surface acoustic wave filters 1003 and 1004 are disposed on the same piezoelectric substrate 1002, it is preferable that the number of a plurality of lines necessary for electrical connection to the outside be minimized and the length thereof be short to achieve further miniaturization. Therefore, a plurality of lines are often commonly connected.
For example, for the surface acoustic wave filter 1003, the first ends of the IDTs 1011b and 1011c connected to the ground potential are commonly connected to a ground terminal 1031 by the above-described line 1013. The ground terminal 1031 is a terminal connected to the external ground potential. The ground terminal 1031 is disposed on the piezoelectric substrate 1002. The ground terminal 1031 is also connected to the line 1023 that is adjacent to the second surface acoustic wave filter 1004. Accordingly, typically, the ground terminal 1031 is arranged in between the plurality of surface acoustic wave filters 1003 and 1004.
As a result, for example, for the first surface acoustic wave filter 1003, the end of the IDT 1011b connected to the ground potential is connected to the line 1013 at a connection point 1032, and the IDT 1011c is connected to the line 1013 at a connection point 1033. Accordingly, the distance of the line section extending between the connection point 1032 and the ground terminal 1031 is longer than the distance of the line section extending between the connection point 1033, which is adjacent to the ground terminal 1031 and connects the second IDT 1011c, and the ground terminal 1031. The IDTs 1011b and 1011c are IDTs connected to the first and second balanced terminals 1006 and 1007, respectively, through the IDTs 1012b and 1012c and the first and second partitioned IDT portions 1012a1 and 1012a2, respectively, of the downstream second-stage longitudinally-coupled resonator type surface acoustic wave filter portion 1012.
The length of a ground line pattern section that achieves electrical connection between the IDT 1012b connected to the first balanced terminal 1006 and the above-described ground terminal 1031 is different from the length of a ground line pattern section that connects the IDT 1012c to the ground terminal 1031. Therefore, for the surface acoustic wave filter 1003, the degree of symmetry of the layout of the line pattern is inevitably decreased.
For this kind of the longitudinally-coupled resonator type surface acoustic wave filter device, a decrease in the degree of symmetry of the electrode and line pattern on a piezoelectric substrate tends to lead to a reduced out-of-passband attenuation in filter characteristics.